What is the difference between Add-on Domain, Parked Domain and Sub Domain?

You have to use the domain tools from your control panel of your server to make all these 3 examples.
Add-on Domain:
Add-on domain is just another domain name but hosted using the space of another main domain. Lets take for example, www.qalbunasrar.com is the main domain and I would like to host my other site, www.klwebmaster.com, using the space of qalbunasrar then www.klwebmaster.com is an add-on domain to the main site. But this new domain should have already been registered and the nameservers settings should point to the nameservers of the main domain. The contents of the add-on domain will reside in a folder under public_html.
The add-on domain can be accessed by different ways:
klwebmaster.com
qalbunasrar.com/klwebmaster
klwebmaster.qalbunasrar.com
In other words, you use the same server or hosting to manage multiple domains. The drawback is that, you only have one control panel for a number of domains in use.

Parked Domain:
This is similar to add-on domain except that instead of hosting another separate website, the parked domain redirects to the main domain. For example, if anybody have their sites registered with all the top level domain names (.com, .net, .org, .edu etc), they can use parked domain to redirect to the main domain. In this case also, the domain should have already been registered and the nameservers settings should point to the nameservers of the main domain.
Sub Domain:
A subdomain is just an extension of the main domain. It is used to create separate regions in the website to easily distinguish separate topics and used for easy access.
The sub domain can be accessed by different ways:
qalbunasrar.com/klwebmaster
klwebmaster.qalbunasrar.com